1. Field of the Invention
The present invention relates to a host based printer which receives compressed image data from a host computer and prints images based thereon, and more particularly to such a printer in which a reception buffer can be omitted and a line buffer is effectively used.
2. Description of the Related Art
Generally, printers receive code data from an external equipment and convert the code data to image data for printing. Because of a time needed for data conversion, there is a limit in increasing printing speed. Recently, host based printers have been extensively used. High performance personal computers are used as host computers which produce compressed dot image data and send the same to the host based printer.
The host based printer includes at least a transmission/reception circuit, a reception buffer, a DMA (direct memory access) controller, a decoding circuit, a line buffer, and a print controller. The transmission/reception circuit transmits data to the host computer and receives data therefrom. The reception buffer temporarily stores parallel compressed dot image data received from the host computer. The DMA controller, connected to both the transmission/reception circuit and the reception buffer, directly accesses the reception buffer to send the compressed dot image data stored therein to the decoding circuit. The decoding circuit decodes the compressed dot image data to reproduce the dot image data. The line buffer temporarily stores one raster line worth of dot image data and outputs serial data for supplying to a print driver. The host based printer is capable of performing a high speed decoding of the compressed dot image data to such an extent that the received compressed dot image data can be almost simultaneously printed. For such host based printers, a large capacity reception buffer is not required but a small capacity reception buffer suffices. However, the reception buffer is requisite and cannot be dispensed with.
Many printers have a resolution selection capability. For example, either a 600 dpi or a 300 dpi printing resolution can be selected. The line buffer has a storage area for storing one raster line worth of dot image data in a high resolution mode. For example, when the resolution is 600 dip, dot image data per one raster line is about 640 byte, and when the resolution is 300 dpi, dot image data per one raster line is about 320 byte.
The conventional host based printer makes use of only a part of the storage area in the line buffer and the rest of the storage area therein remain unused when the printer is changed to a low resolution mode. Therefore, the use of the line buffer is not effective.